1. Field of the Invention
The present invention relates to a semiconductor device having MOSFETs in which semiconductor active layers for forming element active regions have for example silicon-on-insulator (SOI) type substrate separation structures and back side gate electrodes are buried in an insulating layer for substrate separation, and to a method for producing the same. More specifically, the present invention relates to an improvement of the characteristics of a semiconductor device by changing the thickness of the back side gate insulating film.
2. Description of the Related Art
It has been known that complete separation of elements from each other becomes easy by an SOI structure and that suppression of soft-error or latchup peculiar to a complementary metal-oxide-semiconductor (CMOS) transistor becomes possible. Studies have been conducted on increasing the speed and increasing the reliability of CMOS transistor LSIs by an SOI structure having a silicon active layer having a thickness of about 500 nm from a relatively early stage.
Recently it has been learned that by reducing the thickness of the active layer of the SOI structure to about 100 nm and/or controlling also an impurity concentration of the channel to a relatively low state to achieve conditions for substantially the silicon active layer to become fully depleted, further excellent performance such as the suppression of a short channel effect and an improvement of a current drive capability of the MOS transistor can be obtained.
The two leading methods for forming this SOI layer are the separation by implanted oxygen (SIMOX) and using wafer bonding, both of which have become more perfected and have become more well known.
However, these two methods have both advantages and disadvantages at the present time.
In the SIMOX method, the uniformity of thickness of the SOI film is excellent, but in contrast the sharpness of the interface with a buried oxide film is poor, so there remain problems in operating performance, reliability, etc. of the transistor.
On the other hand, an SOI substrate prepared by the wafer bonding method has good characteristics at the buried oxide film interface, but is a complicated process and, in addition, since the SOI film is made thin by polishing, suffers from a problem in controllability of the SOI film thickness if the precision of detection of the end point of the polishing is poor.
The wafer bonding method includes a case where the SOI layer is formed on the entire surface and a case where the SOI layer is separated in the planar direction to form isolated patterns. In the latter case, it is possible to provide a step difference at the substrate to be polished before bonding and use the layer of the insulator (separation region in the plane direction) filled in the depressions as a stop for the detection of the end point of the polishing.
The flow of the process for fabrication of an SOI substrate common to these methods roughly comprises the following four steps:
(a) Planarization and surface treatment of the bonding surfaces
(b) Bonding and annealing
(c) Grinding
(d) Polishing (or selective polishing)
In an SOI substrate prepared in this way, not only can the thickness etc. of the buried insulating film be relatively freely set, but it also becomes possible to form elements and interconnect them on parts to become the active layer of the substrate that can be polished before bonding, and then bury them in the insulating film in advance so as to prepare an LSI having a high degree of integration in which elements are three dimensionally arranged on the two sides of the active layer in the thickness direction.
Further, when fabricating a MOSFET, it is possible to form, buried in the insulating film, not only the front side gate electrode arranged on the front surface side of the silicon active layer via the gate insulating film, but also a second gate electrode. This insulating film buried type gate electrode will be referred to as a xe2x80x9cback side gate electrode.xe2x80x9dWhen using the front side gate electrode for signal input, the short channel effect can be suppressed by control from this back side gate electrode, and the threshold voltage, swing width, or gain of the transistor can be controlled. Further, application to an X-MOS (also referred to as a xe2x80x9cdouble gate MOSxe2x80x9d) using both of the front side gate and the back side gate for signal input to achieve a 2-channel mode transistor becomes possible.
When supplying a bias voltage to the back side gate electrode, in the past there was only the fixed bias method of applying a constant voltage. In recent years the technique of controlling (changing) the bias voltage to be supplied to the back side gate electrode so as to improve the transistor characteristics has been proposed.
With a transistor referred to as a xe2x80x9cdynamic Vth MOSxe2x80x9d employing this bias application method, the value of the voltage supplied to the back side gate electrode is dynamically controlled according to the input signal, making the threshold voltage Vth relatively high and reduce the leakage current when the transistor is off, and making the threshold voltage Vth relatively low and improve the drive capability when the transistor is on.
Accordingly, if using this xe2x80x9cdynamic Vthxe2x80x9d technique, the power supply voltage can be reduced without lowering the operating speed of the transistor and then the leakage current during stand-by can be reduced to enable a reduction of the power consumption of the semiconductor device using the related transistor for an active element.
FIG. 12 shows a sectional view of the principal parts of a semiconductor device of the configuration of the related art. This FIG. 12 shows two transistors having different operation modes, that is, a xe2x80x9cdynamic Vth MOS transistorxe2x80x9d (hereinafter referred to as a xe2x80x9cDV-MOSxe2x80x9d) and a MOS transistor of in a conventional operation mode (hereinafter referred to as a xe2x80x9cCON-MOSxe2x80x9d).
In a semiconductor device 100 shown in FIG. 12, an insulating layer 103 is formed on a supporting substrate 101 via a bonding layer 102.
At the surface side in the insulating layer 103 are formed a silicon active layer 104 for the CON-MOS and a silicon active layer 105 for the DV-MOS separated from each other. Predetermined impurities are added to the silicon active layers 104 and 105 with a relatively low concentration.
In the insulating layer 102 are buried a back side gate electrode 107 facing a bottom surface of the CON-MOS silicon active layer 104 via a back side gate insulating film 106, and a back side gate electrode 109 facing a bottom surface of the DV-MOS silicon active layer 105 via a back side gate insulating film 108 separated from each other. The back side gate insulating films 106 and 108 are made of silicon oxide films having the same thickness. Further, the back side gate electrodes 107 and 109 are made of polycrystalline silicon and doped with predetermined impurities with a relatively high concentration.
On the silicon active layer 104 or 105 is formed a gate electrode 111 of the transistor via a front side gate insulating film 110. Further, on the front surface side in the silicon active layers 104 and 105, though not illustrated, are formed source and drain impurity regions having an LDD structure. An inter-layer insulating film 112 is deposited over the entire surface, the inter-layer insulating film 112 is partially etched through, plugs 113 are buried there, and an interconnection layer 114 is formed thereon.
Summarizing the problems to be solved by the invention, in the semiconductor device 100 of the related art, however, when forming an integrated circuit by mixing the two types of MOSFETs (CON-MOS and DV-MOS) having different operating modes, there was the problem that the electric characteristics of this circuit did not sufficiently draw out the best performances of the transistors.
This is due to the fact that the two types of transistors with the different operation modes each have their own advantages and disadvantages in their characteristics. That is, when looking at the low voltage operation, low power consumption, and other criteria, a DV-MOS is better than a CON-MOS, but a DV-MOS normally uses the back side gate electrode and the front side gate electrode short-circuited, which suffers from the problem that the gate capacity seen from the signal input side becomes large.
When the interconnections are relatively long or many transistors are connected later and have to be simultaneously driven, resulting in an otherwise large load, the increase of the gate capacity does not become so serious a problem and rather the characteristics of the DV-MOS (low voltage operation and low power consumption) are made good use of.
When the interconnections are conversely relatively short or the number of later transistors is small, resulting in a small load, however, use of a CON-MOS having a small gate capacity sometimes makes the circuit characteristics better.
Accordingly, when designing an actual IC, by suitably arranging transistors of different operation modes (CON-MOS and DV-MOS) in one chip according to the load capacity etc., the characteristics of the circuit as a whole are improved.
However, in actuality, no matter how much the designs are optimized in this way, the characteristics of the circuit as a whole are not improved as much as expected.
An object of the present invention is to provide a semiconductor device improving the characteristics of an integrated circuit using a combination of a mixture of a transistor dynamically changing the voltage supplied to the back side gate electrode and a transistor having a constant voltage supplied to the back side gate electrode and a method for producing the same.
The semiconductor device according to the present invention improves the characteristics of the integrated circuit by changing the thickness of the back side gate insulating film between transistors of different operation modes.
According to a first aspect of the present invention, there is provided a semiconductor device comprising a plurality of MOSFETs each including a semiconductor active layer formed on an insulating layer on a supporting substrate, a back side gate electrode facing the surface of the semiconductor active layer at the supporting substrate side via a back side gate insulating film, and a front side gate electrode facing the surface of the semiconductor active layer at the side opposite to the back side gate electrode via a front side gate insulating film, the plurality of MOSFETs comprising a first MOSFET in which the back side gate electrode and the front side gate electrode are insulated and separated from each other and a second MOSFET in which the back side gate electrode and the front side gate electrode are electrically connected, the back side gate insulating film of the second MOSFET being formed thinner than the back side gate insulating film of the first MOSFET.
Specifically, for example, the first MOSFET has the back side gate electrode connected to a supply line of a predetermined voltage and has the front side gate electrode connected to the signal input line, the second MOSFET has the back side gate electrode and the front side gate electrode both connected to the signal input line.
Further, in the semiconductor device according to the present invention, preferably the back side gate electrodes of the plurality of MOSFETs include two types of back side gate electrodes having conductivities different from each other, and the back side gate insulating film contiguous with one back side gate electrode between the two types of back side gate electrodes is formed thinner than the back side gate insulating film contiguous with the other back side gate electrode.
For example, the other back side gate electrode where the adjoining back side gate insulating film is relatively thick comprises a semiconductor material containing boron as an impurity. This thick insulating film effectively prevent boron, which has a large diffusion coefficient, from penetrating through the back side gate insulating film and reaching the semiconductor active layer.
According to a second aspect of the present invention, there is provided a semiconductor device comprising a plurality of MOSFETs each including a semiconductor active layer formed on an insulating layer on a supporting substrate, a back side gate electrode facing the surface of the semiconductor active layer at the supporting substrate side via a back side gate insulating film, and a front side gate electrode facing the surface of the semiconductor active layer at the side opposite to the back side gate electrode via a front side gate insulating film, the plurality of MOSFETs comprising a first MOSFET in which the back side gate electrode is connected to a supply line of a predetermined voltage and the front side gate electrode is connected to a signal input line and a second MOSFET in which the back side gate electrode is connected to a bias switch circuit for switching a value of a supplied voltage between a time of conduction and a time of nonconduction and the front side gate electrode is connected to a signal input line, the back side gate insulating film of the second MOSFET being formed thinner than the back side gate insulating film of the first MOSFET.
In a semiconductor device having such a structure, the second MOSFET in which the back side gate electrode and the front side gate electrode are electrically short-circuited performs a so-called xe2x80x9cdynamic Vthxe2x80x9d operation. That is, for example in an n-channel type, the threshold voltage relatively rises and the leakage current at the off time is reduced when the input signal is at a low level and the transistor is nonconductive, while the threshold voltage relatively falls and the drive capability is improved when the input signal is at a high level and the transistor is conductive. In order to make such an effect larger, it is effective to make the back side gate insulating film thinner so as to enhance the xe2x80x9ccontrollabilityxe2x80x9d of the back side gate electrode with respect to the semiconductor active layer. Accordingly, in this aspect of the present invention, the back side gate insulating film of the second MOSFET is formed relatively thin.
On the other hand, in the first MOSFET in which a constant voltage is supplied to the back side gate electrode, if the back side gate insulating film becomes too thin, a sub-threshold characteristic of the transistor becomes bad, that is, the amount of change of the gate voltage required for changing the sub-threshold current by one order of magnitude (sub-threshold coefficient) becomes large, which is not preferred. Further, when considering the diffusion of the impurity from the back side gate electrode and the insulation characteristics and other aspects of reliability, a thick back side gate insulating film is preferred. Accordingly, in this aspect of the present invention, the back side gate insulating film of the first MOSFET is formed relatively thick.
In this way, in the semiconductor device according to this aspect of the present invention, as a result of the fact that the back side gate insulating film thickness is optimized between the first and second MOSFETs, the characteristics of the integrated circuit using the related MOSFETs are enhanced.
According to a third aspect of the present invention, there is provided a method for producing a semiconductor device comprising: a step of forming a back side gate electrode on a substrate to be polished buried in an insulating layer, a step of bonding the substrate to be polished to a supporting substrate from the insulating layer side, a step of grinding and/or polishing the substrate to be polished from the back side to make the thickness smaller and form the semiconductor active layer, and a step of forming a front side gate electrode on the surface of the semiconductor active layer at the side opposite to the back side gate electrode via the front side gate insulating film, the step for forming the back side gate electrode further including a step of forming a back side gate insulating film having a partially different thickness on the substrate to be polished, a step of forming a plurality of back side gate electrodes on the back side gate insulating film, and a step of depositing an insulating film covering the surfaces of the plurality of back side gate electrodes.
The step of forming the back side gate insulating film includes, for example, a step of forming a first layer of a back side gate insulating film on the substrate to be polished, a step of removing part of the first layer of the back side gate insulating film, and a step of forming a second layer of a back side gate insulating film on the remaining gate insulating film part of the first layer and on the substrate to be polished part exposed by the removal.
Further, preferably, the step of forming the front side gate electrode includes a step of electrically connecting a specific front side gate electrode to a corresponding back side gate electrode according to the thickness of the corresponding back side gate insulating film when simultaneously forming a plurality of front side gate electrodes. For example, when the back side gate insulating film has a first region having a relatively large thickness and a second region having a relatively small thickness, the back side gate electrode and the front side gate electrode formed in an area corresponding to the second region should be electrically connected.
Preferably the back side gate insulating film has a first region having a relatively large thickness and a second region having a relatively small thickness, and, in the step of forming the plurality of back side gate electrodes, a p-type back side gate electrode is formed in the first region and an n-type back side gate electrode is formed in the second region. In this case, the back side gate electrode formed on the first region of the adjoining back side gate insulating film comprises a semiconductor material containing, for example, boron as an impurity. This is because boron penetration can be effectively prevented through the back side gate insulating film and diffusing into the semiconductor active layer by making the back side gate insulating film corresponding to the back side gate electrode containing the boron having a large diffusion coefficient relatively thick.
In such a method of production of a semiconductor device, the step of providing a thickness difference in the back side gate insulating film can be achieved by a combination of lithography and etching. No special step is required.